So Internet Explorer 11 seems to have problems to encode the umlaut from ISO-88591- into UTF-8 (although this setting is activated in the Internet Options -> Advanced -> International).

When writing ue instead of ü it worked.

Update May 3rd 2016:My colleague just found out that this only happens in a recent version of IE11. On Internet Explorer 11 Version 11.0.9600.16521 it was working. Another non-working version is 11.0.9600.17278 and the IE version I tested with was 11.0.9600.18282.